I have a client that wants to essentially duplicate Word's Track Changes feature on a website.

(This feature essentially uses a strikethrough font to indicate text that has been deleted, A different color to indicate text that has been added etc.)

What would be the best WYSIWYG editor for the job? Is there one that already does this?

If not, which editor would most easily be tweaked to accomodate this feature? (I'm assuming you'd need a combination of custom XML tags, custom buttons, and a somewhat tweakable JavaScript implementation (if indeed we're talking JavaScript and not an ActiveX control or Java Applet)).